The stock XB cylinder heads perform very well, especially with a turbocharger forcing extra air through them. I was able to hit over 120hp before I decided to make any changes to them. The first change I made was to open up the intake and exhaust ports a little bit, smoothing out the casting marks and blending the valve seats. I ran the lightly ported stock heads up until the rear rod let go under high boost, but the heads were able to supply over 180hp of charge.
Since more power meant more boost, I decided to lower the compression ratio. I made some changes to the combustion chambers to increase the chamber volume and while I was at it, I installed larger intake and exhaust valves, new springs, and titanium retainers. I used Manley beehive springs and Ti retainers first but the i.d. of the Manley springs would not fit over factory style valve seals so I was forced to use the seals supplied with the spring kit. Later on, I changed to a set of stiffer PAC springs and Ti retainers from Hammer Performance. The factory style valve seals fit well inside the PAC springs.
I sunk both valves deeper in the heads and unshrouded around the seats. I was able to compensate for the deeper valve/ taller valve stem height by installing valves made for a Twin Cam motor with 7mm stems. I used 1.900″ Baisley intake valves, 1.610″ AV&V Exhaust valves, and single bee-hive springs w/ titanium retainers.
I also indexed the new set of NGK DCPR9-EIX plugs to ensure the spark has maximum exposure to the chamber volume.
My latest piston/cylinder setup includes Hammer Performance 3.563″ bore iron-lined aluminum cylinders with a set of their 30 degree reverse dome pistons. With the first attempt, I tried to run a set of off the shelf pistons from them. I sent my heads to them to machine the 30 degree combustion chamber and they supplied me with a set of their 883-1250cc conversion pistons to get the compression ratio I wanted.
Everything fit well but the valve pockets did not clear my valves. The pistons are designed to be used with 883 heads where the valves sit closer together. There was no piston-valve contact when I turned the motor over by hand since the hydraulic lifters were not fully pumped up. I heard a light knocking after I cranked the motor up and sure enough, there was piston-valve contact.
With the 70.5cc chambers (62cc stock) I ran custom CP pistons with 2cc domes and cut for the custom set of Total Seal rings. With 20+ psi of boost, a stainless steel top ring is a good choice to handle the extra heat, and a gapless second ring to ensure minimal blow-by.
Here are the original custom CP pistons:
Here are a few shots of the new OEM cylinders. Finding the part number for just the cylinder and not an entire cylinder/piston kit proved to be difficult. 16954-02A is the H-D number for an individual cylinder. They made only one small change to the top fins, allowing more room to fit the exhaust. You can see the scrape marks on my old cylinder from removing and installing the exhaust.